Sensitivity compensator for scanned displays



Feb. 19, 1963 T. p. KEGl-:LMAN

sENsIfrIvI'TY coMPENsATo'R Foa scANNED DISPLAYS' BY LQW am @rm/@NEW Feb. 19, 1963 T. D. KEGELMAN V 3,078,342

Y SENSITIVITY COMPENSATOR FOR SCANNED DISPLAYS Filed May 10, 1961 2 Sheets-Sheet 2 THQ/was D. A11-'65mm BYMLQAM nrToPA/EY 3,@7342 Patented Feb. i9, i963 dce 3,978,3d2 SENSHEVETY COMPENSATOR FOR SCANNED EESPLAYS Thomas D. Kegelman, West Nyack, NX., assigner to United Aircratt Corporation, East I-mtord, Conn., a

corporation oi Belaware Fded May it?, 196i, Ser. No. 169,155 it? Slaims. (Cl. .W8- 7.2)

My invention relates to a sensitivity compensator for scanned displays and more particularly to an arrangement for producing the eiiect of a constant brightness in an unmodulated scanned display.

In present day scanned display devices the scanning rate is limited by the bandwidth which is transmitted to the device or by limit of the rate of mechanical motion of the sensor. With such limited scanning rates great difficulty is encountered in producing a picture visible to the human eye. Even where a relatively high persistence phosphor is used the resultant picture is not satisfactory owing to the low scanning rate and owing to the relatively poor quality image produced by a high persistence phosphor. lf a high quality image is to be produced a relatively short persistence phosphor is employed. Such a short persistence phosphor, however, requires a scanning rate which is higher than that available in the scanned display. Owing to these facts in present day scanned display devices an unsatisfactory compromise must be made between the quality of the image to be produced and the rate of scan.

I have invented a sensitivity compensator for scanned displays which permits me to use a relatively short persistence phosphor to produce a high quality image while at the same time not requiring an excessively high scanning rate. My sensitivity compensator is simple in its construction and operation. It is readily adapted to use with any type display.

One object of my invention is to produce a sensitivity compensator for scanned displays which compensates for the etlect of decay in the light emitted from the phosphor of the device being scanned.

Another object of my invention is to produce a sensitivity compensator for scanned displays which gives the effect of a constant brightness of an unmodulated display being scanned.

A further object of my invention is to produce a sensitivity compensator for scanned displays which is simple in its construction and in its operation.

Yet another object of my invention is to provide a sensitivity comparator for scanned displays which is readily adapted for use with any type display.

Other and further objects of my invention will appear y from the following description.

ln general my invention contemplates the provision of a sensitivity compensator for scanned displays in which I compare the low rate scanning signal of the device being scanned with the high rate scanning signal of the scanning device to trigger a wave form generator to produce a compensating signal each time the two scanning signals coincide. I apply the compensating signal to the scanning device to increase its sensitivity to compensate for the decrease with time of the brightness of the trace of the scanned device. I subtract the compensating signal from the output of the scanning device to produce a resultant output signal which is not affected by the relatively short persistence of the phosphor of the scanned device.

In the accompanying drawings which form part of the instant specification and which are to be read in conjunction therewith and in which like reference numerals are used to indicate like parts in the various views:

FGURE 1 is a schematic view of one form of my sensitivity compensator for scanned displays.

FlGURE 2 is a diagram illustrating the relationship between the slow scanning signal of the device being scanned and the relatively rapid scanning signal of the scanning device.

FIGURE 3 is a diagram illustrating the relationship between the compensating wave form of my sensitivity compensator for scanned displays and a curve of the decay in brightness of the trace of the device being scanned.

FGURE 4 is a schematic View of an alternate form of my sensitivity compensator for scanned displays.

Referring now to FIGURE 1 of the drawings in one form of my sensitivity compensator for scanned displays the device providing the picture being scanned is a cathode ray tube indicated generally by the reference character le having an envelope l2, a screen lillcoated with a suitable phosphor and deiiection coils i6 to which signals are applied to cause a beam of electrons to make a trace on the screen M. Since the structure of the cathode ray tube is well known in the art, it will not be described in detail. For purposes of explanation I will assume that the cathode ray tube llt) is arranged to produce a rectangular scan in which a beam of electrons moves rapidly from left to right across the screen 14 and moves less rapidly from bottom to topy of the screen. The phosphor which coats the screen 14 in my sensitivity compensator may be a medium persistence phosphor such, for example, as a Pl or P2 phosphor. Deflection circuits indicated by the block i3 in the figure provide voltages for energizing the coils i6 through a channel 20 to cause the beam of electrons to scan the surface of screen 14 in the manner described above. These circuits iii which generally are sawtooth voltage generators deflect the electron beam in a manner well known in the art and for this reason will not be described in detail.

From the structure so far described, it will be appreciated that there appears on the surface of the tube l@ a very bright trace forming a horizontal line travelling from the bottom to the top of the screen 14. This trace is followed by a rapid decrease in brightness resulting from the exponential decay of the phosphor brightness.

'Ihe scanning device which l employ in my sensitivity compensator for scanned `disp-lays is a vidicon tube indicated generally by the reference character 22. The envelcpe 2dof the tube 22 houses an electron gun indicated generally by the reference character 26 including a heater 2%, a grounded cathode 3i), and an anode 32 connected to the terminal 3e of a suitable source of positive potential of a magnitude of, for example, 300 volts. A secondary anode 36 providing a wall screen 33 is connected to the terminal 4@ of a source yof positive potential of 300 volts. A pair of line deflection coils 42 and 44 adjacent the envelope 2.4iare supplied with a suitable voltage for detlecting the beam of electrons from the gun 26 relatively rapidly from left to right across the target to be described hereinafter. A second pair of frame deflection coils, one coil de of which is shown in FIGURE l are adapted to be supplied with a voltage from a scanning circuit 4S through a channel Si? to deflect the beam of electrons along an axis at right angles to the axis of deection provided by coils d2 and 44. A focusing coil S2 surrounds the detlection coils of the vidicon 22 which includes also an alignment coil 54.

The target of the vidicon 22 is made up of a transparent, conductive signal plate 56 the surface of which is coated with a layer 58 of photoconductive material. A biasing resistor di) connected between terminals 66 and 63 provides the target voltage in a manner to be described. A lens itl focuses the picture to be scanned on the signal plate 56.

As is known in the art in :operation of the vidicon tube described, the stream of electrons from the gun 26 is adapted to scan the surface of the photoconductive layer 58. Electrons striking the surface of the photoconduct-ive layer 58 lose their kinetic energy as they strike the target and are therefore at the potential of the cathode which is substantially ground potential. Owing to the biasing voltage applied to the signal plate 56, a considerable electric field exists between the opposite faces of the target. For this reason current ilows parallel to the axis of the envelope 24 at the point where the electron beam strikes the target. The magnitude of this current depends on the signal plate bias and on the target conductivity which in turn is a function of the light incident on the target.

One yof the most useful characteristics of the vidicon tube is that its sensitivity can be controlled over an extremely wide range by changing the target voltage. 'Ihe higher the target voltage the higher is the sensitivity of the tube. I make use of this characteristic in my sensitivity compensator for scanned displays. By changing the target voltage from point to point as the surface of the layer 58 is scanned I achieve a localized control of sensitivity. iFor example, if I reduce the target voltage when the scanning beam of the vidicon 22 is in a particular area of the target, then that area of the vidicon will have low sensitivity. If, however, I increase fthe target voltage when the scanning beam is in another area, then the vidicon has high sensitivity in that area. I make use of this characteristie to compensate for the eiect of decay of the brightness 'of the trace on the screen 14 of the device being scanned'.

Respective channels 72 andl 74 apply the slow speed cathode ray tube scanningvoltage and the high speed vidicon scanning voltage to a comparator '7 6. In the particular embodiment of my invention sho-wn I take the kframe scanning voltage of the cathode ray device and compare it with the framel scanning voltage of thev vidicon 22;

Referring to FIGURE 2, I have shown the frame scanning voltage of the cathode ray' tube by the curve A and I have indicated the vidicon frame scanning voltage by the curve B. From FIGURE Zit will readily be apparent that the rising portions of thev frame scanning voltage of :the vidicon 22 cross the frame scanning voltage A of the cathode ray tube once during each scanning cycle of the vidicon tube. While `the wave form B crosses the wave form A during its fall as well as during its rise, I will consider only the intersections of the two waves during the rising portions of the vidicon scanning signal since these are the only intersections of signilicance. It will be appreciated that an intersection of the two wave forms indi- Cates that position, with reference to the face of the tubeV 10, at which the vidicon beam is at the same position as the cathode ray beam. It will furtherbe appreciated that when the vidicon beam passes the cathode ray tube beam, it is entering into a region of the cathode ray tube which has not been scanned since the last'cycle vof the cathode ray tube scan. That is to say, the vidicon beam is then entering an area of the picture being scanned in which the greatest amount of decay or decrease in brightness has occurred. Recognizing this fact, I have increased the sensitivity o-f :the vidicon at this point by increasing the target voltage.

vIhe comparator 76 -is of any suitable type known in the art which can produce a signal indicating the point of intersection of the two wave forms. For example it may be a High Impedance Transistor Pickoi Circuit such as is shown in the oopending application of Allan T. Kneale, Serial No. 577,587, filed April ll,v 1956, now Patent No. 2,983,830. A channel 78 applies the output of the comparator 76 to a wave form generator 80 adapted to be actuated by the comparator signal. Generator 80 may for example include a gate adapted to couple an RC circuit momentarily to a battery in response to the com,- parator signal. I apply the output of the wave form generator t-o the terminal 68 to provide the target voltage. When the generator is then disconnected from the source of potential the voltage stored therein decays exponentially to produce the desired Wave form. It will readily be appreciated that since the decay in phosphor brightness of the tube 10 is an exponential function that an exponential signal can just compensate for the decay in brightness. That is it can readily be demonstrated that the product of the output of wave forms of generator 80 and the light linx on the surface of the cathode ray tube 10 is a constant.

Since the target voltage output of generator S0 applied to terminal 68 varies, the sensitivity of the vidicon 22 is varied in just such a way as will compensate for the decay in brightness in the trace of the tube 10. By Way of example, I have indicated the output voltage of the wave form generator '30 by the curve C in FIGURE 3 and I have shown the phosphor brightness by the broken line curve D. From an examination of FIGURE 3 with relation to FIGURE 2, it will be seen that from the start of a frame scan of the cathode ray tube the frame scan voltage of the vidicon successively intersects the cathode ray tube voltage atlater points inthe vidicon scan. That is over a frame 'scan of the cathode ray tube beam the vidicon beam coincides with the cathode ray beam at successively later timesin the video cycler until the end of the cathodey ray rtube cycle.

A- vchannel 82 conducts the vidicon' output from terminal 66 to one input terminal of a subtraction circuit 84. I apply the output of the generator 80 to the other Vinput terminal of the circuit 84. This circuit 84, which may be a differential amplier of any type known in the art, subtracts the generator output from the signalv at terminal 66 to produce an output on channel 88 which is the desired signal representing the scanned display. Channel 88 applies the output of the circuit 84 to a buler 86 which may, for example, be a cathode follower. The output of the circuit 86 on a channel 92 is a video output representing thepicture on the face of the tube 10 as if there were no decay in brightness of the phosphor on the screen 14. A suitable video amplifier 94 may be employed to amplify the signal on channel '92 to produce an output signal on channel 96 which may, for example, be applied to any suitable display device.

Referring now to FIGURE 4, I have shown a schematic representation of an alternate form of my invention in which a cathode-ray tube-vidicon assembly includes a cathode ray tube phosphor layer 98 and a vidicon target 100 enclosed in a single envelope 10-2. Respective electron guns 104 and 196 direct their beams on the phosphor layer 98v and on the target 100. In this `form of my invention I have shown only one pair of cathode ray tube deection plates 108 and 110 and only one pair of vidicon deflection plates 112 and 114. It will readily be understood that other sets of plates are provided for the other axis.

A loW rate cathode ray tube deflection circuit 116 and a relatively high rate vidicon deflection circuit 118 provide scanning signals for the respective sets of plates 108 and '110, and 112A and 114. I apply the outputs of the respective circuits 11-6-and 118 to a'diierential amplifier 120 of any suitable type known to the art to produce an output signal for triggering an exponential generator 122 the output of which I apply to the terminal 1124 of the target 100. Each time the differential amplifier 120 indicates that thev video scan intersects the cathode ray tube scan, generator 122 puts out a signal of the type indicated by the wave form C in FIGURE 3 to vary the target voltage to change the sensitivity of the vidicon to compensate for the decay in phosphor brightness of the layer 98.

In operation of my sensitivity compensator for scanned displays lens 70 focuses thev picture from screen 14 on the lsignal plate 56. As the beam of electrons of vidicon 22 scans the photoconductive layer 58, the comparator 76 indicates the times at which the vidicon frame scanning l Y.. Y gli voltage equals the cathode ray tube frame scanning voltage to trigger the Wave form generator 80 to cause a signal to be applied to terminal 68 to change the target voltage to increase the sensitivity of the vidicon to compensate for the decay of brightness of the trace on screen 14. That is, each time the vidicon spot catches up with the cathode ray tube spot the sensitivity of the vidicon is increased to compensate for the fact that the vidicon trace is entering a relatively dark area of the screen 1d which resulted from the decay in phosphor brightness, This action occurs at a later time in each of the successive vidicon scans from the beginning to the end of the cathode ray tube scan. I may arrange my system in the manner shown n FIGURE l or alternatively I may make an assembly of a cathode ray tube and a vidicon in the manner shown in FIGURE 4.

It is to be understood that While I have described my compensator in connection with an orthogonal scanning system, I may as Well apply it to any other type display such, for example, a PPI. For these other than rectangular forms of display the resolved sawtooth waves are fed to the comparator so that the position of the vidicon beam at the time determined by the comparator coincidence pulse is accurately established to coincide with the high brightness scanning line.

It will be seen that I have accomplished the objects of my invention. I have provided a sensitivity comparator for a scanned display in which the output signal is compensated for the effect of decay in brightness of the phosphor of the device being scanned. My system produces an output signal which is as if, for an unmodulated display, a picture of constant brightness for being scanned. My system is extremely simple for the result produced thereby. It is readily adapted to any type display.

It will be understood that certain features and subcombinations are of utility and may be employed without reference to other features and subcombinations. This is contemplated by and is within the scope of my claims. It is further obvious that various changes may be made in details within the scope of my claims without departing from the spirit of my invention. It is, therefore, to be understood that my invention is not to be limited to the specific details shown and described.

Having thus described my invention, what I claim is:

1. A comparator for scanned displays including in combination means providing a display adapted to be scanned, a source of a first deflection voltage for said display providing means, means for scanning said display, a source of a second deflection voltage for said scanning means, means responsive to said deflection voltages for producing a compensating signal and means for applying said compensating signal to said scanning means to vary the sensitivity thereof.

2. A compensator for scanned displays including iny combination means for providing a display adapted to be scanned, a source of a first deflection voltage for said display providing means, means for scanning said display, a source of a second deflection voltage for said scanning means, means responsive to said deflection voltages for producing a comparison signal, means responsive to said comparison signal for generating a compensating signal and means for applying said compensating signal to said scanning means to vary the sensitivity thereof.

3. A compensator for scanned displays including in combination means providing a display adapted to be scanned, a source of a first sawtooth deflection voltage for said display providing means, means for scanning said display, a source of a second sawtooth voltage for said scanning means, said second sawtooth voltage being relatively rapid with respect to said first sawtooth voltage, means responsive to said sawtooth voltages for producing a comparison signal indicating the points at which said second sawtooth voltage coincides with said first sawtooth voltage, means responsive to said comparison signal for generating a compensating voltage and means for ap- 6 plying said compensating signal to said scanning means to vary the sensitivity thereof.

4. A compensator for scanned displays including means comprising a phosphorescent screen for providing a display adapted to be scanned, a source for a first deflection voltage for said scanning display providing means, means for scanning said display, a source of a second deflection voltage for said scanning means, means responsive to said deflection voltages for producing an exponential compensating signal and means for applying said compensating signal to said scanning means to vary the sensitivity Ithereof to compensate for the decay in brightness for said phosphorescent screen.

5. A compensator for scanned displays including in combination means comprising a phosphorescent screen for providing a display adapted to be scanned, a source of a first deflection voltage for said display providing means, means comprising a target for scanning said display, means for applying said display to said target, a source of a second deflection voltage for said scanning means, means respo-nsive to said deflection voltages for producing an exponential compensating signal and means for applying said compensating signal to said target to vary the sensitivity of said scanning means to compensate for decay in brightness of said screen.

6. A compensator for scanned displays including in combination means comprising a phosphorescent screen providing a display adapted to be scanned, a source of a first sawtooth deflection voltage for said display providing means, means comprising a target for scanning said display, means for applying said display to said target, a source of a second sawtooth deflection voltage for said scanning means, means responsive to said deflection voltages for producing a comparison signal indicating the points at which second sawtooth voltage coincides with said rst sawtooth voltage, means responsive to said comparison signal for initiating an exponential voltage at each of said points and means for applying said exponential voltage to said target to vary the sensitivity of said scanning means to compensate for the decay in brightness of said phosphorescent screen.

7. A compensator for scanned displays including in combination means providing a display adapted to be scanned, a source of a rst frame deflection voltage for said display providing means, means for scanning said display, a source of a second frame deflection voltage for said scanning means, means responsive to said deflection voltages for producing a compensating signal and means for applying said compensating signal to said scanning means to vary the sensitivity thereof.

8. A comparator for scanned displays including in combination a cathod ray tube having a screen providing a display to be scanned, a source of a first deflection voltage for said cathode ray tube, a vidicon comprising a target, means for applying said display to said vidicon target, a source of a second deflection voltage for said vidicon, means responsive to said deflection voltages for producing a compensating signal and means for applying said compensating signal to said target to vary the sensitivity of said vidicon to compensate for the decay in brightness of said cathode ray tube screen.

9. A comparator for scanned displays including in combination a cathode ray tube having a screen providing a display to be scanned, a source of a first deflection voltage for said cathode ray tube, a vidicon comprising a target, means for applying said display to said vidicon target, a source of a second deflection voltage for said vidicon means, responsive to said deflection voltages for producing a compensating signal, means for deriving an output Signal from said vidicon and means for subtracting said compensating signal from said output.

10. A scanned display system including in combination an envelope, a target comprising a layer of conductive transparent material and a layer of photoconductive material disposed Within said envelope, a photosphorescent said compensating signal Io said target to compensate' for the decay in brightness of said phosphorescent screen.v

References Cited in the l'e of this patent UNITED STATES PATENTS Pensak Nov.` 12, 1957 Perkins May 24, 1960 

1. A COMPARATOR FOR SCANNED DISPLAYS INCLUDING IN COMBINATION MEANS PROVIDING A DISPLAY ADAPTED TO BE SCANNED, A SOURCE OF A FIRST DEFLECTION VOLTAGE FOR SAID DISPLAY PROVIDING MEANS, MEANS FOR SCANNING SAID DISPLAY, A SOURCE OF A SECOND DEFLECTION VOLTGAGE FOR SAID SCANNING MEANS, 